Pneumatic/hydrostatic riser tension

ABSTRACT

The present invention provides heave compensation through a riser tensioner for a marine riser ( 8 ) supported from a sea vessel ( 100 ) supporting a cylinder barrel ( 11 ) with a mainly vertically arranged cylinder axis ( 110 ). The cylinder is provided with a piston ( 12 ) with an outer diameter corresponding to the cylinder barrel ( 11 ) and arranged coaxially inside the cylinder barrel ( 11 ). The cylinder barrel ( 11 ) has a larger diameter than the riser ( 8 ). The cylinder barrel ( 11 ) has an upper end ( 5 ) communicating with the atmosphere through a vent channel ( 6   c ) from a volume ( 6   a ) above the piston ( 12 ), the vent channel ( 6   c ) and the volume ( 6   a ) arranged for being air-filled, excerting an atmospheric pressure downwards on the piston ( 12 ). The cylinder barrel ( 11 ) has a lower end ( 112 ) arranged for being submerged below the sea surface and being open towards the sea The piston ( 12 ) is subject to a hydrostatic pressure from below from the sea, excerting a net upwards force supporting and balancing the weight of the riser ( 8 ).

FIELD OF THE INVENTION

[0001] The present invention relates to tensioning marine petroleum riser pipes extending between a petroleum producing well at the seabed and a freely floating petroleum production platform at the sea surface.

BACKGROUND TO THE INVENTION

[0002] Freely floating petroleum production platforms, both in the form of monohull production ships and semisubmersible platforms, continuously experience a heave movement due to their interaction with sea waves, wind and current. The riser, which is supported and kept in tension by the platform, should not experience such variation in tension, as this may break, buckle, or in other way harm the riser. Numerous attempts for stable riser tensioning has been made. Active heave compensating devices exist on the market, with hydraulic pistons and wire block systems. Heave sensors connected provide input to an electronic control system for the hydraulic system. One problem involved with such a heave compensation hydraulic system is the wearing-out of wires, and the ability of the control system to react adequately to every heave movement. Such a system is inadvertently complicated and requires a detailed maintenance schedule.

[0003] Norwegian Patent NO 308379 describes a heave compensator constituted by a telescopic section arranged between a lower end of a riser pipe and a wellhead at the seabed, and a non-compensated connection with a ship at an upper end of the riser.

[0004] Norwegian Patent NO 169 027 describes a heave compensation device for a riser arranged between a surface vessel and a wellhead at the seabed, and utilizes a slide joint with a hydraulic cylinder which keeps the riser tensioned when the vessel is subject to heave motion.

[0005] U.S. Pat. No. 4,657,439 is a method and apparatus for supporting the weight of a marine riser by use of a passive tensioning system which utilizes a buoyant member located upon the upper end of the riser. The buoyant member has a vertical slot defined through its side such that the riser may access the member from a lateral direction. Thus the riser may be temporarily supported by other means while changing said buoyant member.

[0006] U.S. Pat. No. 5,758,990 describes air cans for supporting a riser. A buoyancy can is also an element of U.S. Pat. No. 6,161,620 illustrating a “Spar” buoy with an open ended buoyancy can tie which surrounds the upper end of the riser and an upper seal which effectively closes the annulus between the riser and the buoyancy can tube. A load transfer connetion in the buoyancy can assembly then connects the riser to the buoyancy can tube. A pressure charging system communicates with the annulus between the riser and the buoyancy can tube below the upper seal Injecting gas into this annulus then provides support to the riser.

[0007] Norwegian patent NO 307224 describes a more adapted and reliable system with dry tree risers passively supported by buoyancy cans arranged below the most wave- and current influenced zone below the sea surface, the buoyancy cans free to move vertically in a frame extending below the platform thus the dry tree risers stand independently of the platform's vertical motion.

SUMMARY OF THE INVENTION

[0008] The present invention provides heave compensation through a riser tensioner for a marine riser (8) supported from a sea vessel (100) supporting a cylinder barrel (11) with a mainly vertically arranged cylinder axis (110). The cylinder is provided with a piston (12) with an outer diameter corresponding to the cylinder barrel (11) and arranged coaxially inside the cylinder barrel (11). The cylinder barrel (11) has a larger diameter than the riser (8). The cylinder barrel (11) has an upper end (5) communicating with the atmosphere through a vent channel (6 c) from a volume (6 a) above the piston (12), the rent channel (6 c) and the volume (6 a) arranged for being air-filled, excerting an atmospheric pressure downwards on the piston (12). The cylinder barrel (11) has a lower end (112) arranged for being submerged below the sea surface and being open towards the sea. The piston (12) is subject to a hydrostatic pressure from below from the sea, excerting a net upwards force supporting and balancing the weight of the riser (8).

[0009] The principle of the invention is that a submerged wide vertical cylinder contains a piston (12) which is subject to a hydrostatic pressure from below, and atmospheric pressure from above, resulting in a net upwards force. If the piston has a diameter of 3 m, and the depth is 30 m, the upwards net force on the piston is about 210*10⁴ Newton, or a lift force of about 210 metric tonnes. The riser (8) runs centrically through the piston ther riser being hung off on a hang-off ring (3), and also runs through the cylinder top. The top of the cylinder is air-filled and preferrably in communication with the atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is an illustration of the principle of the invention, with a cross-section of a sea vessel supporting a riser by means of a piston in a vertically arranged cylinder barrel supported by said vessel according to the invention.

[0011]FIG. 2 is an illustration of a preferred embodiment of the invention, with a section of a pontoon of a semisubmersible platform, the pontoon being provided with a riser tensioning system with a cylinder barrel being closed at its' upper end and having an air vent communicating with the atmosphere.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0012] In a preferred embodiment of the invention, heave compensation is provided through a riser tensioner for a marine riser (8) supported from a sea vessel (100) supporting a cylinder barrel (11) with a mainly vertically arranged cylinder axis (110), The cylinder barrel is supported by a connector (14) to a pontoon (101) of a semisubmersible platform (1). The cylinder is provided with a piston (12) with an outer diameter corresponding to the cylinder barrel (11) and arranged coaxially inside the cylinder barrel (11). In a preferred embodiment, the piston is situated at a depth about 30 metres below sea surface, but other depths may be preferred according to the draught of the actual ship or semisubmersible platform in use. A periphery of said piston (12) is provided with one or more piston rings (9) against the inner wall of said barrel (11). The cylinder barrel (11) has a larger diameter than the riser (8) The cylinder barrel (11) has an upper end (5) communicating with the atmosphere through a vent channel (6 c) from a volume (6 a) above the piston (12), the vent channel (6 c) and the volume (6 a) arranged for being air-filled excerting an atmospheric pressure downwards on the piston (12). The cylinder barrel (11) has a lower end (112) arranged for being submerged below the sea surface and being open towards the sea. The piston (12) is arranged essentially below the sea surface (6 g), and is subject to a hydrostatic pressure from below from the sea, excerting a net upwards force supporting and balancing the weight of the riser (8). In the preferred embodiment, the diameter of the barrel and the piston is 3 metres, but smaller or larger diameters may be desirable, depending on the submerged weight of the riser.

[0013] For practical purposes, the cylinder barrel (11) itself does not have to extend all the way from the pontoon to above the surface. Said upper end (5) of the barrel (11) is provided with a cylinder top (50) with a riser seal (6) said vent channel (6 c) having a lower channel end (61) in said volume (6 a) above the piston (12) and an upper channel end (62) in said atmosphere (6 e).

[0014] Said vent channel (6 c) may in alternative embodiments comprise a valve or choke valve (not illustrated) for keeping a slight under- or overpressure in the air or gas above the piston (12), in order to adapt to particular heave conditions, e.g. long periods if that should be required, but in the basic embodiment the air moves freely through channel (6 c), maintaining atmospheric pressure above the piston (12).

[0015] The piston (12) has in the preferred embodiment an axial opening (13) with a diameter corresponding to said diameter of said riser (8), with a rim shoulder (131) of said axial opening (13) arranged for supporting a riser hangoff ring (3) on said riser (8). Further in the preferred embodiment of the invention, said riser seal (6) has a diameter allowing passage of an outer casing (7) around said riser (8), said outer casing (7) extending from said piston (12) to above said sea surface (6 g) said outer casing (7) comprising said vent channel (6 c).

[0016] In the preferred embodiment of the invention, the outer casing (7) is provided with safety valves (10) in its lower end, just above the top surface of the piston, the safety valves arranged for releasing more of said air (6 f) from said air volume (6 a) to the atmosphere (6 e) should the pressure in said air volume (6 a) exceed a given pressure limit (Pr) if the platform should drop fast or deep surge of a wave during the heave movement of the semisubmersible platform or ship. Arranging one-way valves through the piston (12) has been considered, but this idea has been rejected due to the fact that if one such one-way valve through the piston should fail, loss of riser tension would occur.

[0017] In case of a water leakage from the sea into the air-filled volume (6 a), the piston is provided with a small drain sump (21) in the top surface of said piston, said drain sump (21) feeding a water lift pump connected to a water drain line (20) extending up through said outer casing (7) for draining undesired water from the bottom of said air-filled volume (6 a).

[0018] Said piston and barrel needs a full diameter only in the vertical operating range of said piston, and the barrel may be narrower above a desired operating range. Also, the channel from the air space above the piston needs not go only in the outer casing, but may be a relatively narrow separate channel to free atmosphere above the water line.

[0019] In a preferred embodiment, the tensioner according the invention can have one or more spare pistons (12′) for safety reasons, arranged immediately below said piston (12), inside said barrel (11), and one or more spare lift pumps (2) arranged connected to said drain sump (21) and to said water drain line (20). 

1. Tensioner (0) for a marine riser (8) supported from a vessel (100) floating at the surface (6 g) of a sea (6 f), comprising a cylinder barrel (11) having an upper end (5) and a lower end (112), supported by said vessel (100) and having a cylinder axis (110) mainly for vertical arrangement, and having a piston (12) with a diameter being larger than a diameter of said riser (8); said piston (12) separating a variable volume (6 a) filled with air (6 d) communicating with the free atmosphere (6 e) through a vent channel (6 c) above said piston (12), from a water-filled variable volume (6 b) below said piston (12), said lower end (112) of said barrel (11) arranged for being submerged below the sea surface and being open towards the sea (6 f), with said piston (12) being submerged to a depth essentially below said sea surface (6 g); said air (6 d) of said volume (6 a) excerting pressure downwards on said piston (12), said piston (12) being subject to a hydrostatic pressure from below from the sea (6 f), excerting a net upwards force for supporting and balancing the weight of said riser (8).
 2. Tensioner according to claim 1, said upper end (5) at the barrel (11) having a cylinder top (50) with a riser seal (6), said vent channel (6 c) having a lower channel end (61) in said volume (6 a) and an upper channel end (62) in said atmosphere (6 e);
 3. Tensioner according to claim 1, said piston (12) having an axial opening (13) with a diameter corresponding to said diameter of said riser (8), with a rim shoulder (131) of said axial opening (13) arranged for supporting a riser hangoff ring (3) on said riser (8)
 4. Tensioner according to claim 2, said riser seal (6) having diameter allowing passage of an outer casing (7) around said riser (8), said outer casing (7) extending from said piston (12) to above said sea surface (6 g), said outer casing (7) comprising said vent channel (6).
 5. Tensioner according to claim 1, a periphery of said piston (12) being provided with one or more piston rings (9) against the inner wall of said barrel (11).
 6. Tensioner according to claim 1, said piston being provided with safety valves (10) arranged in the lower part of said outer casing (7) for releasing parts of said air (6 f) from said air volume (6 a) should the pressure in said air volume (6 a) exceed a given pressure limit (Pr) during a deep surge of heave movement.
 7. Tensioner according to claim 4, with a drain sump (21) in the top surface of said piston (12), said drain sump (21) feeding a water lift pump (2) connected to a water drain line (20) extending up through said outer casing (7) for draining undesired water from the bottom of said air-filled volume (6 a).
 8. Tensioner according to claim 2, with a spare piston (12′) arranged immediately below said piston (12), inside said barrel (11).
 9. Tensioner according to claim 7, with one or more spare lift pumps (2) arranged connected to said drain sump (21) and to said water drain line (20). 